Heated Nozzle Unit for the Moulding of Plastics Materials

ABSTRACT

A nozzle has a tubular metal core which forms a central injection duct, and a cylindrical, outer, lateral surface. A heater/diffuser device includes a metal heat diffuser of tubular cylindrical shape with an open annular cross-section, mounted around the cylindrical surface of the core. The diffuser has: a cylindrical internal surface fitting the cylindrical surface of the nozzle core, two facing longitudinal free edges which are spaced apart circumferentially, are free of one another, and define between them a longitudinal space extending along a generator line of the cylindrical surface of the diffuser, and a recessed, channel-like seat formed in a surface of the diffuser element. A resistor is housed in the channel-like seat. The distance between the longitudinal free edges is such that, when the nozzle unit is heated by the supply of electrical current through the resistor, the diffuser is free to expand thermally by extending in the circumferential direction around the nozzle without moving radially away from the outer surface of the nozzle core.

The present invention relates to a heated nozzle unit for the mouldingof plastics materials.

Conventionally, an injection nozzle for the moulding of plasticsmaterials comprises a cylindrical, tubular steel core which forms acentral longitudinal injection duct for injecting the molten plasticsmaterial through one or more injection holes into a moulding cavity of amould. An electrical resistor is wound as a helix around the tubularcore to heat the plastics material which flows through the injectionduct and to keep the parts of the nozzle which are affected by the flowof material at a controlled temperature such as to prevent the materialfrom solidifying. The windings of the resistor are usually closertogether in the region close to the injection hole, which is closer tothe moulding cavity and therefore tends to cool more quickly than thecentral regions of the nozzle. A capillary thermocouple detects thetemperature of the nozzle in the vicinity of the injection-hole. Theheat supplied by the resistor tends to accumulate in the central regionof the nozzle where higher temperatures are reached than in the regionof the injection hole; at times, these higher temperatures are notpermissible for the type of plastics material being processed whichinstead should be kept within a fairly limited temperature range toprevent degradation of the plastics material. Thus, the resistor isactivated as soon as the thermocouple detects a temperature below apredetermined minimum value in the region of the injection hole but,even though the temperature of the injection duct in the central regionsis acceptable, that temperature rises as a result of the switching-on ofthe resistor until it exceeds a maximum permissible value for thematerial.

In these known solutions, spiral resistors of rectangular cross-sectionare mostly used in order to increase the contact area between theresistor and the tubular nozzle core around which the resistor is wound.However, the contact area naturally constitutes only a fraction of theoverall surface of the resistor so that most of the heat generated bythe resistor is not actually transmitted to the nozzle but is dissipatedinto the surrounding mould and is thus lost. In fact the mould is inturn cooled in order to keep the walls of the moulding cavity at as lowa temperature as possible in order to speed up the solidification of themolten material and thus shorten the moulding cycles.

In order to dissipate the heat from the central portion of the nozzleand to distribute the heat more uniformly along the injection duct, ithas been proposed to incorporate the resistor in a tubular metaldiffuser element which is fitted externally on the tubular nozzle core.According to this solution, a channel-like seat is formed in the outersurface of a cylindrical tubular element and the resistor is insertedtherein. However, here again, excessive heat dispersal occurs from theouter surface of the resistor towards the surrounding mould; moreover,direct contact (and hence direct transmission of the heat by conduction)is not achieved between the resistor and the tubular nozzle core.

The object of the present invention is therefore to provide an improvedheated nozzle unit, addressing principally the problems of:

-   -   heating the injection duct of a nozzle uniformly within a        predetermined and limited temperature range;    -   preventing accumulation of heat and hence excessively high        temperatures in the central portions of the nozzle, and    -   optimizing the consumption of electrical energy for the supply        of the resistors as well as reducing the amount of heat        dissipated thereby towards the mould in which the nozzles are        mounted.

These and other objects and advantages, which will be understood furtherfrom the following description, are achieved by a heated nozzle unithaving the characteristics defined in the appended claims.

The characteristics and the advantages of the invention will becomeclear from the detailed description of an embodiment thereof which isgiven with reference to the appended drawings which are provided by wayof non-limiting example and in which:

FIG. 1 is a cross-section through a heated nozzle unit according to theinvention, comprising a nozzle and a heater/heat-diffuser device,

FIGS. 2 and 3 are a plan view and a cross-section taken on the lineIII-III, respectively, of a plate-like semi-finished product for theformation of a heater/diffuser device of the type shown in FIG. 1,

FIG. 4 is a plan view of the plate of FIG. 2 on which a resistor isfitted,

FIGS. 5 to 7 are views showing schematically three separate bendingsteps for the formation of the heater/diffuser device,

FIG. 8 is a cross-section through the heater/diffuser device,

FIG. 9 is an enlarged view of a detail of FIG. 8,

FIG. 10 is a cross-section taken on the line X-X of FIG. 1, and

FIGS. 11 and 12 are two plan views similar to FIG. 2 of two furtherembodiments of the heat-diffuser according to the invention.

With reference now to FIG. 1, an injection nozzle generally indicated 10for the moulding of plastics material is mounted in a mould 20 with amoulding cavity 21.

The nozzle 10 comprises a steel body 11 which is formed integrally witha cylindrical tubular core 12 with an outer lateral surface 13.

The tubular core 12 has a central longitudinal injection duct 14 whichextends between an upper region 15 for the input of the molten materialas far as a lower region 16 in which a conventional tip 17 which formsthe end portion of the injection duct 14 is inserted. A capillarythermocouple indicated 19 extends as far as the vicinity of the lowerregion 16 of the injection duct 14 in order to detect the temperature inthe vicinity of the region in which the plastics material is injected inthe molten state into the moulding cavity.

A tubular cylindrical diffuser 30 is mounted on the lateral surface 13of the core 12; the diffuser 30 has an open annular cross-sectiondefining a central cavity fitting the lateral surface 13 of the core.For reasons which will be explained below, as shown in FIGS. 8 and 10,the diffuser 30 has facing and spaced apart free longitudinal edges 30e, 30 f which define between them a longitudinal space 30 g extendingalong a generator of the cylindrical surface of the diffuser 30.

The diffuser 30 is made of a metal or a metal alloy having high thermalconductivity, for example, brass, copper, or aluminium and, when mountedon a nozzle as shown in FIG. 1, extends substantially along the entirelength of the steel core 12.

The diffuser 30 has, on its internal cylindrical surface 30 d, arecessed channel-like seat 32 which houses an electrical resistor 40 forheating the core 12 directly. The channel 32 follows a path whichextends around the lateral surface of the core in a manner such that theresistor 40 transmits heat to the core 12 uniformly, cooperating withthe diffuser 30. By virtue of its high thermal conductivity, thediffuser 30 also ensures that a substantially uniform temperature ismaintained along the core 12 and prevents heat accumulating andexcessively high temperatures arising in the central regions of thenozzle.

A method for the manufacture of the heater/diffuser device 30 and forits mounting on the nozzle 10 is as follows.

Starting with a flat metal sheet, a substantially rectangular flat platesuch as that indicated 30 a in FIGS. 2 and 3 is produced. The plate 30 ahas a height h corresponding to the length of the nozzle core 12 onwhich the diffuser is fitted and a width w slightly less than thecircumference of the cylindrical surface 13 of the core. The thickness tof the plate is selected such that the channel 32 which is subsequentlyformed in one of the two faces of the plate is suitable foraccommodating a wire resistor 40 the diameter of which is preselected independence on the desired electrical characteristics. For example, aplate thickness of about 2 mm may be used to house a 1 mm diameter wire.In the preferred embodiment shown in the drawings, the two opposedlongitudinal edges 30 e, 30 f of the plate 30 a are inclined alongplanes which converge towards a face 30 d of the plate which will facetowards the interior of the nozzle in use.

The internal face 30 d of the plate 30 a is then processed, for example,by milling, electrical-discharge machining, or other known processes toform a channel 32 having a substantially C-shaped or U-shapedcross-section such as to accommodate a resistor 40 exactly, or with apredetermined clearance. Alternatively, the step of the formation of thechannels 32 may be performed on the starting sheet before it is cut intoa number of plates corresponding to the number of channels.

The resistor 40 is then fitted in the channel 32 (FIG. 4). To ensuredirect contact with the surface of the nozzle core in the mountedcondition, the surface of the resistor is level with the face 30 dhaving the channel 32 or projects slightly beyond that face.

The plate 30 a with the resistor fitted is then placed in a firstbending tool P1 (FIG. 5) in which the two opposed longitudinal edges 30e, 30 f of the plate are bent in the direction in which the face 30 dfaces. The plate is then placed in a second bending tool P2 (FIG. 6) inwhich a C-shape or a U-shape is imparted to the plate. Finally, in athird bending tool P3 (FIG. 7), the body 30 is given its final opentubular cylindrical shape with a central longitudinal axis x parallel tothe opposed edges 30 e, 30 f.

A cylindrical forming tool P4 is used during this bending step and isplaced in contact with the internal surface 30 d. The tool P4 has adiameter slightly smaller than that of the cylindrical surface 13 of thenozzle core 12 on which the diffuser is to be fitted.

During the bending steps shown in FIGS. 6 and 7, the bending of thediffuser 30 advantageously also causes a slight convergence of thefacing surfaces 32 a, 32 b of the channel 32, at least in the portionsin which the channel 32 extends parallel to the longitudinal axis of thediffuser. The convergence of the facing surfaces 32 a, 32 b towards thecentre of the diffuser (FIGS. 8 and 9) has the effect of gripping theresistor 40 in the channel 32. Naturally, in this case, the shapes andsizes of the channel 32 and of the resistor 40 will have to be selectedprecisely to achieve this clamping effect.

As indicated schematically in FIG. 10, the capillary thermocouple 19 ishoused and restrained firmly in the space defined between the two edges30 e and 30 f and the outer surface 13 of the tubular core 12. Thethermocouple 19 has a diameter greater than the maximum distance betweenthe free edges 30 e and 30 f. This maximum distance is that which ispresent when the nozzle is cold.

It is important to point out that, when the diffuser 30 with theincorporated resistor 40 is mounted on the nozzle in a “cold” condition,the longitudinal edges 30 e and 30 f are spaced apart circumferentiallyby a predetermined distance “d”, for example, of about 0.5 mm. By virtueof this arrangement, when the nozzle is heated by the supply ofelectrical current through the resistor 40, the diffuser 30 is free toexpand thermally by extending in the circumferential direction aroundthe nozzle and does not tend to expand radially and hence to move awayfrom the outer surface 13 of the nozzle, as would happen if the diffuserwere a tubular element with a closed annular cross-section or if theedges 30 e and 30 f were restrained rigidly relative to one another.According to the invention, therefore, the diameter of the internal face30 d of the diffuser remains substantially unchanged during the coolingand heating cycles. This ensures that the resistor 40 is always indirect contact with the surface 13 of the nozzle core 12. It should alsobe noted that, during the heating stage, the steel core 12 (which has aclosed annular cross-section) also expands radially and this radialexpansion of the nozzle therefore promotes improved contact with theresistor 40 in spite of the fact that the steel of which the nozzle ismade has a lower thermal expansion coefficient than the brass of whichthe diffuser 30 is preferably made.

In the condition of maximum heating, the opposed edges 30 e and 30 f maystill be slightly spaced apart or may be in contact with one another butwithout this leading to radial expansion of the diffuser and henceappreciable movement of the face 30 d of the diffuser, and hence of theresistor 40, away from the outer surface 13 of the nozzle.

As mentioned above, during the step of the bending of the plate 30 a, atool P4 of slightly smaller diameter than the cylindrical surface 13 ofthe nozzle core 12 is preferably used. Once bent to its final curvedshape, the internal face 30 d of the diffuser thus has a diameter which,in the free or undeformed condition, is slightly smaller than theoutside diameter of the core 12. The diffuser 30 can thus be mounted onthe core 12 with slight radial interference by resilient opening-out ofthe diffuser by means of a suitable tool (not shown) in order to fit thediffuser onto the nozzle core. By virtue of this interference fit, thediffuser is clamped resiliently on the nozzle and further ensurescontinuous contact between the nozzle and the resistor.

As will be appreciated, in contrast with the conventional diffusersdiscussed in the introductory portion of the description, according tothe present invention, the resistor 40 is in direct and continuouscontact with the nozzle core 12, apart from the difference in thethermal expansion coefficients of the materials of which the diffuserand the nozzle core are made. The homogeneous distribution of theresistor around the internal cylindrical surface of the diffuser ensuresuniform transmission of heat to the injection duct, particularly in itsregion closest to the injection hole. In addition, since the resistor 40contacts the cylindrical wall 13 of the nozzle core directly, it ispossible to reach and maintain the prescribed temperature in theinjection channel with a lower consumption of electrical current. Theportion of the surface of the resistor 40 which is in contact with thewall of the channel 32 transmits heat to the diffuser 30 rather thandispersing it into the mould. Experimental tests carried out by theApplicant have shown that the invention permits a saving of electricalenergy of 60% or more.

The invention is not intended to be limited to the embodiments describedand illustrated herein, which should be considered as examples of theconstruction of the nozzle unit. Rather, the invention may be modifiedwith regard to the shape and arrangement of parts and constructional andoperational details. For example, as shown in FIGS. 11 and 12, the pathfollowed by the channel 32 along the internal surface of the diffusermay vary in dependence on the type of nozzle, on the dimensions thereof,and on operating requirements. FIGS. 11 and 12 show two examples inwhich the channel 32 (and therefore the resistor 40) is distributed moredensely in the upper and lower end regions of the diffuser and lessdensely in the central region to prevent accumulation of heat in thatregion.

1. An injection nozzle unit for the moulding of plastics materials,comprising: a nozzle with a tubular metal core forming a centrallongitudinal injection duct and a substantially cylindrical, outer,lateral surface, a heater/diffuser device which includes: a metal heatdiffuser of tubular cylindrical shape with an open annularcross-section, mounted around the cylindrical surface of the core, witha substantially cylindrical internal surface fitting the cylindricalsurface of the nozzle core, with two facing longitudinal free edgeswhich are spaced apart circumferentially, are free of one another, anddefine between them a longitudinal space extending along a generatorline of the cylindrical surface of the diffuser, and with a recessed,channel-like seat formed in a surface of the diffuser element, and aresistor housed in the channel-like seat, the distance between thelongitudinal free edges being such that, when the nozzle unit is heatedby the supply of electrical current through the resistor, the diffuseris substantially free to expand thermally by extending in thecircumferential direction around the nozzle without moving radially awayfrom the outer surface of the nozzle core.
 2. A nozzle unit according toclaim 1, wherein the diffuser is mounted on the outer surface of thenozzle core with radial interference so as to grip the core resiliently.3. A nozzle unit according to claim 1, wherein the facing longitudinaledges define, with the outer surface of the nozzle core, a space forhousing a capillary thermocouple having a thickness greater than themaximum distance between the free edges and when the nozzle is cold. 4.A nozzle unit according to claim 3, wherein the two opposed longitudinaledges of the diffuser are inclined along planes parallel to a centrallongitudinal axis of the central nozzle, converging towards the outsideof the nozzle.
 5. A nozzle unit according to claim 1, wherein therecessed, channel-like seat is formed in an internal cylindrical surfaceof the diffuser element that is in contact with the outer cylindricalsurface of the nozzle core.
 6. A nozzle unit according to claim 1,wherein the resistor housed in the channel-like seat is arrangedsubstantially level with or in contact with the outer surface of thenozzle core in order to transmit heat directly thereto.
 7. A nozzle unitaccording to claim 1, wherein the channel-like seat has a substantiallyC-shaped or U-shaped cross-section in order to accommodate the resistorexactly, with a predetermined minimum clearance, or with slightinterference.
 8. A nozzle unit according to claim 7, wherein thechannel-like seat has facing surfaces which converge slightly towardsthe centre of the diffuser in order to restrain the resistor in theseat.